The present invention relates generally to braking systems and more particularly to braking systems for heavy duty vehicles, such as agricultural tractors. Because of the size of these vehicles, it has become customary to provide some type of power braking system so that a large braking force can be applied to a given wheel.
One of the more recent developments in braking systems for such vehicles is what is known as a power assist system wherein the braking pressure of the fluid received by the brake is greater than the inlet pressure of the fluid from the source. This type of system is presently incorporated into agricultural tractors manufactured by the assignee of the present invention.
Because of the numerous hydraulic systems associated with vehicles of this type, it becomes necessary for a single source of pressurized fluid to be utilized for supplying the fluid to various hydraulic systems. For example, in the hydraulic system incorporated into existing agricultural tractors the same pressurized fluid source supplies fluid to the hydraulically actuated brake as well as the hydraulic steering system for the vehicle and, in some instances, may also supply fluid to other hydraulic circuits, such as the transmission.
When more than one system is supplied with hydraulic fluid from a single source, there is usually some type of priority mechanism incorporated into the system to insure that one of the hydraulic circuits receives pressurized fluid whenever necessary. While the amount of fluid that is needed for actuating a hydraulic brake is small, it is absolutely essential that such fluid be available whenever necessary and that fluid also be available for the steering system, if both circuits have a common fluid source.